Many systems are known for measuring the heart rate of a person. Such systems require that the patient be undressed in the area of a pulse point, or at least be lightly clad thereat. In some military situations or emergency situations a person becomes injured such that a heart rate measurement is desired while the person is wearing heavy protective clothing that cannot easily or quickly be removed. An example of where such a situation arises is in firefighting.
It is well known that continuous wave (cw) microwave radar, commonly referred to as doppler radar, can be used to sense and measure motion. Basically, the operation of cw microwave radar can be understood by considering the consequence of combining, or mixing, a sample of the transmitted signal with the miniscule amount of signal reflected back from some object, commonly referred to as the target. Depending upon the phase relations between the sample and the reflected signals, the result will be usually a minute increase or decrease in the detectable signal level at this point. Except in specialized interferometer type measurement apparatus where it may be used to determine position, this fixed offset level is barely discrenible and not expecially useful.
If, however, the target is in motion relative to the transmitter, then the small offset level changes and a small time-varying component, often called the doppler signal, is added to the mixed signal. It is this alternating signal that is processed in most simple cw microwave radars.
For speedometers or traffic control radars, the frequency of the alternating signal, the doppler frequency, is proportional to relative speed and is processed to display miles per hour. For motion detectors, such as automatic door openers, the presence of an alternating signal is sufficient to trigger the desired response.
For measuring the periodicity of some form of regular reciprocating motion, the waveform obtained from the detected mixer output can be filtered and viewed on an oscillograph or processed and counted against a known timebase to compute revolutions per minute or, as in the area of interest of this invention heartbeats per minute.
It is possible to use doppler type radar to detect the movement of the heart if the patient can stay still and stop breathing while the measurement is being taken. However, a patient that can exercise that much control over his physical movement probably doesn't have to have his heart rate determined while wearing protective clothing.
It is a known fact that higher microwave frequencies do not penetrate human tissues as well as lower frequencies. This fact is utilized in the instant invention.